Frequency synthesis is used to generate 'signals at one or more precise frequencies. These signals may then be used to perform frequency conversion in radio frequency (RF) sensor and communication systems. Frequency synthesis may be provided by several different methods. Of concern in frequency synthesis, are the phase, frequency and amplitude stability of the generated signal. Since the generated signal may be used as a local oscillator signal for frequency up-conversion or down-conversion, instability in the signal results in decreased signal-to-noise performance.
One method of frequency synthesis involves the generation of a multiple tone lightwave signal that can be converted into a RF carrier or local oscillator signal. In this method, optical heterodyning is used to create a sum or difference beat frequency from two optical wavelength tones. The sum or difference beat frequency is detected by a photodetector or similar apparatus to generate an RF carrier or local oscillator signal. However, the stability of the beat frequency signal is limited by the relative stability of each of the optical wavelength tones.
In, R. Logan, R. D. Li, Final Technical Report for DARPA Program, “Radio Frequency Photonic Synthesizer” an optical heterodyning circuit, shown in FIG. 1, is disclosed. This circuit contains a mode locked laser 100, two DFB lasers 102, 104, an optical splitter 122, two optical circulators 114, 116, an optical combiner 124, two photodetectors 110, 112 connected to power-control feedback circuits 106, 108, and two Mach-Zender modulators 118, 120. The components of this optical circuit are interconnected using optical fiber. As the figure shows, the optical comb generated by a master laser (which in this case is a mode-locked laser) is split by a power splitter 122 and then is sent, via optical fibers, to injection-lock a pair of slave lasers, DFB-laser-1102 and DFB-laser-2104. Each of these slave lasers would become injection-locked to a line of the optical comb if the spacing or detuning between the free-running laser's lasing wavelength and that line of the optical comb is less than the laser's lock-bandwidth, a wavelength range determined by the injected power Pi. In particular, temperature tuning (˜0.1 nm/° C.) was used to change the slave lasers lasing wavelengths, so that they were tuned to within the injection-locking bandwidth of two selected lines in the incident optical comb. In between the master and slave lasers, a pair of Mach-Zender modulators 118, 120 is used as variable attenuators to provide a way to adjust the injected optical power Pi. The optical outputs from the Mach-Zender modulators 118, 120 are fed to port 1 of two optical circulators 114, 116. The optical signals from the two slave lasers 102, 104 are used for heterodyning and are obtained through port 2 of those optical circulators 114, 116. The slave laser outputs from port 2 of the two circulators 114, 116 are subsequently combined by combiner 124 and then sent to a photodetector (PD3) for heterodyning.
As shown in FIG. 1, the photonic components (optical isolators, Mach-Zender modulators for controlling injected optical power, optical circulators, and optical combiner) used to generate the optical outputs were all fiber-pigtailed/connectorized. Such use of optical fiber to interconnect multiple discrete components results in sensitivity to environmental disturbances such as mechanical or temperature perturbations, which ultimately cause reduced phase stability. In particular, the use of optical fiber links to interconnect the various components of FIG. 1 makes it difficult to keep the optical path lengths of arms I and II equal or balanced over the long term. Since the length of a section of optical fiber typically can be cleaved to an accuracy of only several millimeters, it is difficult to control and balance the overall path lengths of arms I and II to an accuracy of better than 1-2 cm. Also, because the fibers were not co-located physically, environmental perturbations (such as temperature or mechanical disturbances) could cause differential phase fluctuations between the optical inputs to the slave lasers. Theoretically, the phase noise (δφI and δφk, |i−k|=n) of the ith and kth lines in the optical comb generated by a mode-locked laser are given by:δφI=(δφo)I+iδφR  (1)δφk=(δφo)k+kδφR  (2)
In equation 1 and 2 (δφo)I,k and δφR are, respectively, the phase fluctuations in the mode-locked laser and the RF-source driving the mode-locked laser. For a high-quality RF source and for lines in the optical comb that correspond to higher order modes of the mode-locked laser, the magnitude of δφo is much larger than δφR. The phase noise of the microwave signal (fs=nfR) generated via optical heterodyning of diodes 1 and 2 is then given by;                δφ21(f)=[δφI(f)−δφk(f)]+(residual due to the process of injection locking).If the optical path lengths of arms I and II are identical, then δφo, i.e. (δφo)I=iδφo) common to the injection locking of diodes 1 and 2, and δφI(f)−δφk(f)=nδφR(f), where |i−k|=n. We thus obtain the minimum value of [δφI(f)−δφk(f)], and the best phase-noise for the microwave signal fs. From the above discussion, it is obvious that one needs to keep the optical path lengths of 1 and 2 equal and stable to attain the lowest phase noise for the microwave signal fs that we generate via optical heterodyning. Likewise, optical phase stability in the output paths of DFB lasers 1 and 2 (e.g. from the output of the DFB laser through the associated optical circulator and to the optical combiner) translates into amplitude stability for the optically synthesized microwave signal. In the prior art described above, it was difficult to maintain differential phase stability between the fiber links of arms I and II. This weakness, in turn, deters the field deployment of the photonic synthesizer described in ref. 1. Therefore, there is a need in the art for a photonic synthesizer which can increase phase stability in different environments.        